feat: install most dependencies in cacheable layer, before adding main

Instead of requiring the whole stack to be reinstalled whenever a specific version (e.g. main) causes a cache fail, this aims to have stable default version with a second-stage installation of the potentially-modified versions of eicrecon and juggler. This should allow all builds to cache the first installation in virtually all cases, leading to faster builds.

containers/jug/dev.Dockerfile

@@ -132,7 +132,7 @@ RUN --mount=type=cache,target=/ccache,id=${TARGETPLATFORM}              \
     --mount=type=secret,id=mirrors,target=/opt/spack/etc/spack/mirrors.yaml \
     <<EOF
 set -e
-export CCACHE_DIR=/CCACHE_DIR
+export CCACHE_DIR=/ccache
 source ${SPACK_ROOT}/share/spack/setup-env.sh
 find /var/cache/spack/blobs/sha256/ -atime +7 -delete
 spack buildcache update-index eics3rw